1. Field of Invention
The invention relates to communication technology, and more particularly to the decoding of broadband wireless communication signals.
2. Description of Related Art
Broadband wireless technology will have an important role in the future evolution of advanced global telecommunications. Providing multimedia access to portable terminals requires a high-speed but low-complexity wireless technique to combat fading and multipath propagation, while minimizing the power requirement of the wireless modem. Multicarrier modulation, which divides the total signal bandwidth into many narrowband subchannels to be transmitted in parallel, is an attractive known technique to overcome the problem of multiple time-delayed arrivals of symbols, or multiple delay spread. However, a multicarrier signal with a large number of subchannels has a large peak-to-average power ratio, and thus requires the use of highly linear transmit amplifiers which are not power-efficient.
Another known candidate wireless technology is an equalized single-carrier system using quaternary phase shift keying (QPSK) and a decision feedback equalizer (DFE). This technique relies on adaptive signal processing to undo the effects of the delay spread. While thought to be difficult to implement for channels with severe dispersions (e.g., delay spreads on the order of several tens of the symbol period), low-complexity DFE techniques are known in the art which employ a reduced number of feedforward filter taps and a training-free feedback filter. These can lead to low training overhead and practical hardware realization.
However, another challenge with the DFE approach is to overcome error propagation. In practical wireless systems, error correction coding is an important technique needed to achieve reliable coverage in cellular systems with high frequency reuse. Most coding techniques are designed to correct random errors. A DFE, however, produces errors that are bursty in nature. This is due to the fact that the DFE relies on delay-free hard decisions (before decoding) to cancel intersymbol interference (ISI) in subsequent data symbols, so decoding errors tend to trigger strings of improper cancellation. The resulting error propagation severely limits the achievable coding gain of any error correction technique.
A number of schemes have been proposed in the art to remedy the error propagation problem. Those include: (i) Transmitter preceding (see M. Tomlinson, "New automatic Equalizers Employing Modulo Arithmetic", Electronics Letters, vol. 42, pp. 138-139, March 1971; and R. Price, "Nonlinearly Feedback-Equalized PAM vs. Capacity, for Noisy Filter Channels," IEEE ICC'72, Philadelphia, Pa., June 1972, pp. 22-12 to 22-17; the disclosures of each being incorporated herein by reference); (ii) use of interleaving to introduce delay necessary for reliable feedback (see M. V. Eyuboglu, "Detection of Coded Modulation Signals on Linear, Severely Distorted Channels using Decision-Feedback Noise Prediction with Interleaving," IEEE Transactions on Communications, vol. 36, pp. 401-409, April 1988; K. Zhou, J. G. Proakis and F. Ling, "Decision-Feedback Equalization of Time-Dispersive Channels with Coded Modulation," IEEE Transactions on Communications, vol. 38, pp. 18-24, January 1990; and T. Wang and C. L. Wang, "On Adaptive Decision-Feedback Equalization of Intersymbol Interference Channels in Coded Modulation Systems", IEEE Transactions on Communications, vol. 44, pp.1 1404-1412, November 1996; the disclosures of each being incorporated herein by reference); and (iii) use of tentative or soft decisions for feedback (see O. Agazzi and N. Seshadri, "On the Use of Tentative Decisions to Cancel Intersymbol Interference and Nonlinear Distortion With Application to Magnetic Recording Channels," IEEE Transactions on Information Theory, vol. 43, pp. 394-408, March 1997; S. H. Muller, W. H. Gerstacker and J. B. Huber, "Reduced-state Soft-Output Trellis-Equalization Incorporating Soft Feedback," IEEE GLOBECOM '96, London, UK, November 1996, pp. 95-100; and H. Igarashi, K. Ueda, K. Murakami and T. Fujino, "Performance of a Soft-Output Adaptive Equalizer Combined with Soft-Decision Metrics Generator based on Differential Detection," IEEE VTC '97, Phoenix, Ariz., May 1997, pp. 700-704; the disclosure of each being incorporated herein by reference).
Each of these solutions has drawbacks. Transmitter preceding requires a priori channel information which is not available in many wireless systems. The interleaving approach requires the DFE to be implemented in a noise-predictive form, with independent adaptations of the feedforward and feedback filters. From a performance perspective, a noise-predictive structure is not desirable for a low-complexity DFE with a short feedforward filter, as known in the art. Known proposed techniques based on soft decisions are prohibitively complex to implement.